The functions of the cerebral cortex depend on highly interconnected and dynamic microcircuits composed of two types of neurons, glutamatergic excitatory neurons that propagate the signals through the various stages of processing and GABAergic interneurons (INs) that regulate this information flow and sculpt cortical circuits. Signal processing in the cortex depends critically on the activity of these interneurons. This Program Project is focused on the characterization of a population of GABAergic INs whose size and breadth have been previously underestimated. These INs express the ionotropic serotonin receptor 5HT3a and largely originate during development from the caudal ganglionic eminence (CGE). Preliminary studies show that 5HT3aR INs represent about 30% of the total IN population in somatosensory cortex, and that they are concentrated in superficial layers, where they represent the largest IN population. This suggests that they are important in the processing of information in the associative layers of cortex. 5HT3aR INs are heterogeneous, but they are uniformly and potently modulated by serotonin and acetylcholine via ionotropic receptors. The Program Project will investigate the roles of cortical 5HT3aR INs on the development and function of the cerebral cortex. The PPG will consist of three research projects and two cores (an Administrative Core and a Molecular and Transgenic Core) to support the work of the three projects. Project I (by Gordon Fishell), will elucidate the mechanisms that determine the development of the 5HT3aR INs population. It will investigate the genetic program that governs the differentiation of 5HT3aR INs precursors, their connectivity throughout development and the role of activity on their maturation in the cortex. Project II (by Bernardo Rudy) will advance our understanding of the role of 5HT3aR INs in cortical function. Specifically, it will test the hypothesis that they are important in context-dependent sensory processing. It will investigate the functional connectivity of 5HT3aR INs, use photostimulation of channelrhodopsin-expressing cholinergic and serotonergic axons to investigate their modulation by these subcortical inputs and two photon targeted recordings in vivo to investigate the activity of 5HT3aR INs during different behavioral states and their response to motor activity and sensory stimulation. Project III (by Wen-Biao Gan) will investigate the location, structure and plasticity of the synapses 5HT3aR INs make on layer V pyramidal neurons and the role of these INs in regulating the activity and structural dynamics of pyramidal cells during learning and memory formation.